DFT Study of Hydrogen Adsorption on Palladium Decorated Graphene

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DFT Study of Hydrogen Adsorption on Palladium Decorated Graphene Ignacio Lopez-Corral,† Estefanía German,† Alfredo Juan,*,† María A. Volpe,‡ and Graciela P. Brizuela† † ‡

Instituto de Física del Sur, Universidad Nacional del Sur-CONICET, Av. Alem 1253, 8000 Bahía Blanca, Argentina Planta Piloto de Ingeniería Química, Universidad Nacional del Sur-CONICET, camino de La Carrindanga Km. 7, 8000 Bahía Blanca, Argentina ABSTRACT: The adsorption of several molecular and dissociative dihydrogen systems on a Pd-decorated graphene monolayer was studied using the density-functional theory. Our calculations show that the most favorable graphene-supported coordination structure is similar to the PdH2 complex in vacuum, where the H-H bond is relaxed but not dissociated. We also computed overlap populations corresponding to bonds and atomic orbital interactions in order to study the evolution of the chemical bonding. During the decoration process with Pd, we detected a weakening of C-C bonds close to the adsorption site and the formation of strong C-Pd bonds, coming from interaction between C 2pz and Pd 5s, 5pz, and 4dz2 orbitals. After H2 molecule adsorption, the H-Pd bond is formed by the H 1s orbital overlap with the Pd 5s orbital, but this interaction became stronger during the atomic hydrogen adsorption. The objective of this work is to contribute to the understanding of the hydrogen uptake of Pd-doped graphene surfaces.

1. INTRODUCTION Since their discovery in 1991,1 carbon nanotubes (CNTs) have become one of the most promising nanoscale materials. In effect, due to their unusual physical, chemical, and mechanical properties,2 CNTs have possible practical industrial applications in multiple and diverse fields. The basic structural unit of CNTs is the one-dimensional single-walled (SW) CNT, which can be considered as a graphene sheet rolled up into a compact tube. Graphene, the single-atom-thick two-dimensional carbon sheet with the same structure as the individual layers in graphite, has also been the focus of attention considering the recent isolation of a single monolayer of graphene by Novoselov et al.3 The remarkable properties of graphene sheets could facilitate their application in areas like hydrogen technology,4 electronics,5 and sensing.6 In particular, a lot of interest in the use of nanostructured carbon materials for hydrogen storage has been generated since the first experimental report of hydrogen adsorption on SWCNTs by Dillon et al.7 Early experimental researches8,9 have suggested that bare CNTs could act as an efficient hydrogen storage medium as a result of their cylindrical structure and high surface-to-volume ratios, but subsequent studies10-14 have originated controversial conclusions because only small hydrogen storage capacities (lower than 1 wt %) were reached using pristine CNTs at ambient conditions, which does not satisfy the U.S. Department of Energy requirements (>6 wt %15) for practical applications. Nonetheless, several experimental works have found that the incorporation of alkali16-18 or transition metal (TM)19-33 atoms can make more active nanostructured carbons for hydrogen adsorption. Alkali metals are more lightweight elements, and so r 2011 American Chemical Society

could help to achieve a larger gravimetric density of hydrogen. Theoretical reports34-37 suggest that s-block metals adsorbed on fullerenes bind up to six H2 molecules per metal atom at high coverage, giving a gravimetric density of ∼9.5 wt %. However, it appears that during the doping reaction the alkali atoms can block some carbon microporous adsorption sites, leading to a decrease in the specific surface area and in the maximum adsorption capacity observed at low temperature.16 As a result, for example K-intercalated graphite samples exhibit modest enhanced hydrogen adsorption relative to the pristine material.17 Moreover, it was found that moisture drastically increased the weight gain by reactions with (or adsorption on) alkali metals adsorbed on graphite or CNTs,18 and so special caution needs to be taken to remove any moisture contamination from hydrogen. On the other hand, Pd is frequently chosen because the hydrogen storage capacity in carbon materials increases significantly after Pd functionalization,21-25 even at room temperature. Thus, Zacharia et al.24 have reported a hydrogen capacity of 2.9 wt % in Pd-coated multiwalled (MW) CNTs at 298 K and 1.67 MPa, which is approximately 40 times higher than that of the pure MWCNTs, and recently Lipson et al.25 found that SWCNTs encapsulated by thin Pd layers onto a Pd substrate present a large storage capacity of 8-12 wt %. Other TMs have also been utilized to increase the capacity of carbon materials respect to hydrogen adsorption, including Pt26 or Ni.27,28 Yang et al. have explained this activation by means of a mechanism involving hydrogen dissociation on TM particles followed by Received: October 20, 2010 Revised: December 22, 2010 Published: February 22, 2011 4315

dx.doi.org/10.1021/jp110067w | J. Phys. Chem. C 2011, 115, 4315–4323

The Journal of Physical Chemistry C atomic hydrogen spillover and adsorption on the carbon surface.38 They found that providing a TM catalyst supported on active carbon to work as a primary hydrogen source increases the hydrogen uptake of CNTs,29,30 especially when carbon bridges are formed between the supported catalyst and the secondary high surface area carbon material that acts as a hydrogen receptor.31-33 Theoretical studies also demonstrated that TM-doped carbon nanomaterials have very high hydrogen storage capacity.28,39-45 Theoretical reports by Yildirim et al. showed that a single Ti atom adsorbed on a (8,0) SWCNT39 or on a C60 fullerene40 binds strongly four H2 molecules, yielding a hydrogen density of ∼8 wt % for a uniform Ti coverage. They have explained the increased hydrogen uptake of Ti-decorated carbon materials by the formation of artificial metal-carbide-like structures, on the basis of Dewar, Chatt, and Duncanson coordination46 to bind Ti atoms to carbon rings and a Kubas interaction47 to bind dihydrogen species. Similar ab initio studies of hydrogen adsorption on TMdoped carbon-based structures showed that other light TMs, such as Ni, Sc, and V, can bind up to five H2 molecules per metal atom,28,41,42 while heavy TMs, like Pt and Pd, interact strongly with only two H2 molecules.43 Interestingly, Xiao et al.44 have recently found that physisorption of hydrogen on a Pd-decorated (8,0) SWCNT is notoriously enhanced near Pd adatoms, so five hydrogen molecules can bind to two adjacent Pd atoms and the hydrogen storage capacity reaches to 2.88 wt % at large Pd coverage, in concordance with previous experimental measurements.24 Nevertheless, the functionalization of carbon nanomaterials for hydrogen storage through Pd decoration still needs to be explored. In a previous study,45 we presented a qualitative description of the main bonding interactions during the adsorption of atomic and molecular hydrogen onto Pd-doped graphene sheet and SWCNTs using a semiempirical tight-binding method. We found in all cases that strong Pd-H bonds are formed especially during atomic hydrogen adsorption, which suggests that the hydrogen storage process involves the dissociation of H2 molecule on the decoration points and the bonding between resultant atomic hydrogen and the carbon surface, in agreement with experimental evidence. No significant differences were found using graphene and SWCNTs models, so we assumed that it is possible to achieve a good estimation of the local adsorption process that occur on decorated wide-diameter SWCNTs through a graphene sheet approach. To the best of our knowledge, there are no firstprinciples density-functional theory (DFT) studies of atomic and molecular hydrogen adsorption on Pd-decorated graphene. The theoretical reports previously mentioned have considered only the adsorption of H2 molecules on functionalizated CNTs. The aim of the present work is to study the interaction between hydrogen and a Pd-decorated monolayer of graphene by means of DFT calculations, including several molecular and dissociative coordination structures. First, we investigated the preferential adsorption geometries, and then examined the changes in the electronic structure and chemical bonding during the adsorption process. The frontier molecular orbital occupation in the hydrogen fragment is also reported.

2. COMPUTATIONAL DETAILS Our calculations of hydrogen adsorption on Pd-doped graphene layer were performed within the framework of firstprinciples DFT,48,49 implemented with the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA)

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Table 1. Optimal Geometrical Parameters and Binding Energies (Eb) for Free H2, PdH, and PdH2 Systems system

dPd-H (Å)

dH-H (Å)

— H-Pd-H (deg)

Eb (eV)

H2 PdH PdH2h I II III IV

1.54 (1.53)c 1.69 (1.68) f 1.75 (1.77) f 1.54 (1.52) f 1.66 (1.64) f

0.75 (0.746)a 0.90 (0.88) f 0.80 (0.79) f 1.76 (1.74) f -

31.0 (30.4) f 69.8 (69.8) f 180.0

4.638b (4.520)a 2.630d (2.386)e 0.936g (0.837) f 0.406g (0.295) f 0.761g (0.642) f unstable

Experiment.60 b Eb = 2E(H) - E(H2). c Experiment.61 d Eb = E(H) þ E(Pd) - E(PdH). e Experiment.62 f DFT calculations.66 g Eb = 2E(H) þ E(Pd) - E(PdH2). h PdH2 structures I-IV are presented in Figure 2.

a

code.50,51 We adopted the generalized gradient approximation (GGA) to treat the electronic exchange and correlation effects, as described by Perdew-Burke-Ernzerhof.52 In all procedures, a split-valence double-ζ basis set of localized numerical atomic orbitals was used, including polarization functions (DZP), with an energy shift of 50 meV and a split norm of 0.15.51,53 An energy cutoff of 150 Ry for the grid integration was chosen to represent the charge density.50 The basis set superposition error (BSSE) was eliminated by adding ghost atoms to the calculation on the isolated adsorbate. Ghost atoms possess basis functions as normal but do not otherwise affect the calculation (no projectors, compensation charges, and so on), thereby ensuring that the same degrees of freedom are available to the wave functions in any calculation. This procedure is called the counterpoise method and is described in detail in refs 4 and 54. Standard norm-conserving Troullier-Martins pseudopotentials,55 in their fully separable form,56 were used to describe the electron-ion interaction. The following electronic states were considered as valence in the pseudopotential description of the atoms: Pd:5s14d9; C:2s22p2; and H:1s. The atomic positions were fully relaxed, without any symmetry constraint, using the conjugated gradient (CG) minimization technique to obtain the optimized structures. Spin-polarized calculations were performed in all cases. We studied the bulk fcc Pd in order to verify the reliability of the present method, using an 8 8 8 Monkhorst-Pack k-point set. The optimization process gave a lattice constant equal to 3.89 Å and a cohesive energy of 3.82 eV/atom, which are in good agreement with the experimental values (3.89 Å and 3.94 eV/ atom57). Previous pseudopotential GGA-DFT calculations underestimated the cohesive energy as well.58,59 We also studied the bond lengths and dissociation energies of the H2 and PdH molecules as another test of the method. As can be seen in Table 1, the agreement with the experimental results60-62 is reasonable. Following a previous work by Rojas and Leiva,63 the graphene sheet was represented by means of a periodic model, with a tetragonal unit cell consisting of 24 C atoms in the (x,y) plane, forming a honeycomb arrangement. The in-plane cell parameters were 7.39 and 8.53 Å, selected after previous tests, while the lattice constant in the z direction normal to the carbon layer was 20 Å, in order to minimize the interaction between adjacent cells. A Monkhorst-Pack k-point set of 4 4 1 was used for the graphene-based systems. The minimization process gave a lattice parameter of 2.459 Å, in good accord with the experimental data.64 First, we considered the adsorption of single Pd atoms on the graphene monolayer. The Pd-decorated graphene system was modeled using one Pd atom per unit cell, adsorbed on different 4316

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The Journal of Physical Chemistry C

Figure 1. Scheme of the unit cell. The three adsorption sites studied are indicated.

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Structures I and II correspond respectively to side-on and end-on approach modes, both with a relaxed H-H bond, while side-on complexes III and IV have this bond dissociated. We determined the H-H and Pd-H bond lengths (dH-H, dPd-H) and the HPd-H angle ( — H-Pd-H) for each free and adsorbed PdH2 system, and the geometrical parameters zC-Pd, dC-Pd, and Δz during the adsorption process on each site onto graphene. The supported complexes were also rotated along the z direction (see the rotation angle θ in Figure 2b). The binding energy for hydrogen adsorption on the Pd-functionalizated graphene was obtained from the expression Eb ðHn Þ ¼ EðC24 Þ þ EðPdÞ þ EðHn Þ - EðHn =Pd=C24 Þ

Figure 2. Geometry and atom labeling considered for the calculations: (a) PdH system and (b) PdH2 complexes.

high-symmetry adsorption sites: the top site directly above a C atom, the hollow site at the center of a hexagon, and the bridge site at the midpoint of a C-C bond (see Figure 1). As suggested in the literature,65 we determined several geometrical parameters for each adsorption site after relaxation: zC-Pd, defined as the perpendicular distance between the Pd atom and the average z coordinate of the C atoms of the graphene layer; dC-Pd, the distance from the Pd atom to its nearest C atoms; and Δz, the deformation of the graphene surface, measured by the maximum deviation in the z direction of the distorted C atoms from the average position of the graphene sheet. The binding energy of Pd was calculated as Eb ðPdÞ ¼ EðC24 Þ þ EðPdÞ - EðPd=C24 Þ where E(C24) is the total energy of the bare graphene layer, E(Pd) is the total energy of the free Pd atom, and E(Pd/C24) is the total energy of the Pd-functionalizated graphene. Therefore, a positive binding energy corresponds to an attractive interaction. Next, we investigated the interaction between hydrogen and the Pd-doped graphene layer. In a preliminary study, we analyzed a vertical approach of atomic hydrogen on the mentioned sites on decorated graphene, determining the geometrical parameters previously defined (zC-Pd , dC-Pd , Δz) and the distance between the Pd and H atoms, dPd-H (see Figure 2a). Then we studied the adsorption of dihydrogen on Pd-decorated graphene, considering different PdH2 coordination structures. Both side-on and end-on approach modes were taken into account during the formation of the PdH2 systems, as displayed in Figure 2b, according with previous reports of unsupported complexes.66,67

where n is 1 or 2, according to the number of H atoms, E(C24) is the total energy of the bare graphene layer, E(Pd) is the total energy of the free Pd atom, E(Hn) is the total energy of the free H atom or H2 molecule, and E(Hn/Pd/C24) is the total energy of the H atom or H2 molecule adsorbed on the Pd-doped graphene. Finally, we analyzed the changes in the chemical bonding process, using the optimized adsorption geometries previously established. The bonding evolution, including atomic orbital interactions, was studied by means of two conceptual tools: density of states (DOS) and crystal orbital overlap population (COOP). The DOS curve is a plot of the number of orbitals per unit volume per unit energy, while the COOP curve is an energyresolved plot of the overlap population-weighted density of states. Analyzing the DOS and COOP curves, we may evaluate the extent to which specific states contribute to a bond between a pair of atoms. Integration of the COOP curve up to the Fermi level gives the overlap population (OP), which can be considered as a measure of bonding between two specified atoms or orbitals.68 Complementary tests were achieved applying the atom superposition and electron delocalization tight-binding (ASED-TB) scheme, which is a semiempirical method but has been successfully utilized in the study of different TM-C structures.69-73 Details about this theory and the YAEHMOP package74 implementation to the graphene/Pd/H2 system can be found in a previous work.45

3. RESULTS AND DISCUSSION 3.1. Adsorption Geometries. The optimization results for the free and graphene-supported systems studied in this work are listed respectively in Tables 1 and 2, with indication of the binding energies (Eb) and the corresponding geometrical parameters. The interaction between the H2 molecule and the Pd atom is well-known, so we studied the isolated PdH2 systems to test the accuracy of our method. Comparing the binding energies in Table 1, it can be seen that the most stable structure is the system I, the so-called Kubas complex,47 with the H2 molecule not dissociated and the H-H bond toward the Pd atom. The second in order of stability corresponds to the structure III, with a dissociated H-H bond. Our results indicate that the dissociative adsorption of H2 on the Pd atom is 0.18 eV less favorable than the molecular process. The linear Pd-H-H complex II, which comes from the end-on approach mode, is very weakly bonded to the Pd atom and more unstable than the other molecular state. Finally, the linear H-Pd-H isomer IV has remarkably longer Pd-H bonds and a considerable negative binding energy, and so this complex becomes unstable. Also from Table 1, we can see the 4317



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Table 2. Optimal Geometrical Parameters and Binding Energies (Eb) for Pd, PdH, and PdH2 Graphene-Supported Systems system

site

graphene/Pd

top

bridge

hollow

graphene/Pd/H

graphene/Pd/H2

I

II

III

IV

a

zC-Pd (Å)

dC-Pd (Å)

2.18

2.18

(2.21)a

(2.10)a

(2.16)b

(2.14)b

2.14

2.25

(2.21)a

(2.18)a

(2.16)b

(2.22)b

2.02

2.47

(2.03)a (2.04)b

(2.46)a (2.47)b

top

2.18

2.18

bridge

2.14

2.25

hollow

2.02

top

dPd-H (Å)

dH-H (Å)

— H-Pd-H (deg)

θ (deg)

Eb (eV)

-

-

(1.044)a

1.077 -

-

(1.04)b 1.082 -

-

-

-

(1.081)a (1.09)b 0.894

-

-

-

-

(0.852)a (0.91)b

1.59

-

-

-

2.923

1.59

-

-

-

2.960

2.47

1.58

-

-

-

2.540

2.20

2.20

1.74

0.85

28.3

30

1.878

bridge

2.15

2.26

1.75

0.84

27.8

60

1.846

hollow

2.13

2.53

1.74

0.86

28.4

30

1.740

top bridge

2.20 2.14

2.20 2.26

1.81 1.81

0.77 0.77

0.1 0.0

0 0

1.354 1.358

hollow

2.02

2.47

1.80

0.77

0.0

0

1.194

top

2.16

2.16

1.75

0.85

28.0

30

1.890

bridge

2.25

2.35

1.75

0.85

28.1

60

1.862

hollow

2.24

2.62

1.73

0.87

29.0

30

1.708

top

2.53

2.53

1.65

3.30

176.1

30

unstable

bridge

2.41

2.50

1.65

3.30

175.1

60

unstable

hollow

2.68

3.01

1.66

3.32

170.8

30

unstable

DFT calculations.65 b DFT calculations.75

same trend Eb(I) > Eb(III) > Eb(II) > Eb(IV) as reported for Efremenko et al. using Gaussian94.66 Regarding the Pd-doped graphene surface, Table 2 shows that the most favorable location for a single Pd atom is the bridge site, at the center of a C-C bond, followed by the top site. In effect, the binding energy of a Pd atom located on a C atom differs in only 5 meV respect to adsorption on the bridge site, while the weakest interaction takes place at the hollow location. For the studied adsorption sites, no significant deformation of the graphene surface (Δz < 0.1 Å) was found. These results are in agreement with previous GGA-DFT calculations performed by Chan et al.65 and recently by Cabria et al.75 (see Table 2). Using GGA-DFT methods, Maiti and Ricca76 also indicated the bridge and top sites as the most stable locations for a single Pd atom adsorbed on graphene, while Durgun et al.77 and Xiao et al.44 have found that the adsorption of isolated Pd atoms on a (8,0) SWCNT occurs preferably on a C-C bond parallel to the tube axis, with a binding energy of 1.67 eV and a C-Pd distance of 2.12 Å. According to Maiti and Ricca,76 our binding energies for Pd atoms adsorbed on graphene are approximately 40% smaller than those reported on SWCNTs, because the finite curvature imparts higher reactivity to CNTs. Considering now the interaction between hydrogen and the Pd-decorated graphene, it can be seen in Table 2 that the bridge site is the preferential adsorption position in presence of atomic H, followed by the top location. The hollow site has the smallest binding energy and the deformation of the graphene layer is small. Although DFT-based data about the H adsorption on Pd-doped graphene are still not available, we have obtained a similar behavior for this system by means of semiempirical

calculations.45 Interestingly, adsorption of atomic H does not modify substantially the positions of Pd atoms, which supports the idea of a significant Pd-substrate interaction. As a consequence, the Pd-H bond in carbon-supported complexes is a little weaker than that of the isolated PdH molecule, while the Pd-H distance elongates from 1.55 Å (see Table 1) until 1.59 Å (see Table 2). Finally, we analyzed the adsorption of molecular hydrogen onto the Pd-functionalizated graphene surface. Considering the obtained binding energies shown in Table 2, the most stable supported PdH2 coordination structures belong to the complexes I and III, which present almost the same geometrical parameters when they are adsorbed. In effect, during the adsorption process the H-H distance of the PdH2 system III shortens from 1.76 Å (see Table 1) to 0.85-0.87 Å (see Table 2), and at the same time the H-Pd-H angle varies from approximately 70 to 28-29. Hence the complex III switches from a free dissociated state to a supported molecular structure comparable to the TM-dihydrogen complexes synthesized by Kubas,47 which results in a more stable structure in the presence of the graphene layer. Table 2 also shows that the adsorption of the linear Pd-H-H system II is less stable than the preceding complexes, whereas the complex IV is extremely unstable. All these findings are in agreement with the previous GGA-DFT work by Xiao et al.44 These authors did not find atomic H or hydrogen dissociation during the adsorption of two H 2 molecules on a Pd-decorated (8,0) SWCNT. Moreover, they mentioned that, when a single H2 molecule was adsorbed on the bridge site of the Pd-doped SWCNT, the H-H bond was significantly weakened and its length increased from 4318



Figure 3. COOP curves for C-Pd and H-Pd bonds in the Pddecorated graphene monolayer, for the bridge (a-e) and top (f-j) sites.

0.75 Å (in the free H2 molecule) to 0.84 Å, while the C-Pd distance was stretched from 2.12 to 2.17 Å. The same behavior was also reported by Dag et al.43 As can be seen in Table 2, the binding energies obtained for each PdH2 structure in the bridge and top sites are similar, the hollow location being the least stable adsorption location. In all cases, the distortion of the graphene layer as a consequence of the PdH2 adsorption is small, and the rotation of the complexes along the z direction, described by the angle θ, has a minor effect on the binding energy. In fact, the changes in the binding energy of the PdH2 isomers with θ are smaller than 1.5%, for all adsorption geometries considered. Although these results are an indication of a restricted adsorbate-surface interaction, the binding energies become higher on the adsorbed PdH2 systems than on the free-standing complexes (compare Table 1 vs Table 2). Thus, we could conclude that the carbon support provides an additional stabilization to the PdH2 coordination structures. 3.2. Bonding Analysis. We explored the evolution of the chemical bonding during the adsorption processes on graphene by means of COOP curves and OP values, belonging to selected pairs of atoms in the preferential adsorption geometries. According to the preceding results, in each case there are only small differences between the bonding energies corresponding to the different systems adsorbed on the bridge and top sites, and so we examined bonding changes on both locations. In the case of the free and graphene-supported PdH2 system, we studied the Kubas complex, with a relaxed and not dissociated H-H bond. The computed COOP curves for C-Pd and H-Pd bonds during the adsorption of H and H2 on the Pd-decorated graphene monolayer are shown in Figure 3, following the C atom labeling indicated in Figure 1. In these plots, the contributions to the right of the vertical axis are bonding between the specified atoms, and those to the left are antibonding. With respect to the adsorption of Pd on the bridge site, the COOP curves corresponding to C1-Pd bond show bonding contributions except to a small antibonding region near to the Fermi level (EF), both without (Figure 3a) and with hydrogen (Figure 3, b and c). However, the C1-Pd COOP curve when atomic hydrogen is adsorbed (Figure 3b) additionally displays a noticeable bonding peak near -14 eV due to the H-Pd interaction, while the COOP curve after H2 adsorption (Figure 3c) only presents very small bonding peaks at lower energies. Similar characteristics can be found in the COOP curves belonging to the H-Pd bond in the presence of H (Figure 3d) and H2 (Figure 3e), which suggests

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that a stronger H-Pd interaction is developed when atomic hydrogen is adsorbed on the Pd-doped graphene. COOP curves coming from the Pd decoration located on the top site are comparable, but they seem to indicate that in this case the C1-Pd interaction (Figure 3, f, g, and h) is stronger than that obtained during the adsorption on the bridge site. In order to complete the previous analysis of the bond evolution, we next evaluated OP values for H-H, H-Pd, C-C, and C-Pd bonds in all the studied systems, which are listed in Table 3. The results show that the C-C bonds near the adsorption sites are weakened after the Pd decoration process due to the formation of new C-Pd bonds. In effect, the OP values for the C1-C2, C1-C3, and C1-C4 bonds decrease about 2.7% when the Pd atom is located on the top site, while the adsorption at the midpoint of the C1-C2 bond reduces its OP 3.3%. Moreover, the results given in Table 3 demonstrate that the weakening of the C-C bonds decreases significantly when their distance respect to the decoration point increases. For example, the OP values belonging to the C1-C3 and C1-C4 reduce only 1.5%, approximately, during the adsorption on the bridge site, while the C2-C5 and C2-C6 bonds present a similar behavior. In agreement with the previous COOP discussion, the adsorption of the Pd atom directly above C1 originates a strong interaction, but the C-Pd OP values developed on C2, C3, and C4 are in this case very small. Contrarily, the bridge position implicates both C1 and C2 with individual meaningful OP. As a result, a single Pd atom adsorbed on the bridge site of graphene is more stable than that located in the top site, matching our energetic analysis. When hydrogen is adsorbed on the Pd-functionalizated graphene, new H-Pd bonds are formed and the strength of C-Pd and C-C bonds changes significatively. However, no C-H OP is found during the adsorption process. According with the COOP analysis, different bonding performances can be found in the presence of atomic or molecular hydrogen. Table 3 shows that during H adsorption the OP value for the H-Pd bond decreases only 1.2% with respect to the isolated PdH system, while the value when H2 is adsorbed on Pd decreases 28.3% and the H-H OP bond increases up to 9%, approximately, compared with the free, not dissociated PdH2 complex. In this way, we could say that the Pd-H interaction developed in the presence of atomic hydrogen is stronger than that originated during the molecular adsorption process. A similar finding can be found on analyzing the evolution of the C-Pd and C-C bonds. In effect, as can be seen in Table 3, the OP values for the C-Pd bonds when H is adsorbed decrease about 15.5 and 16.2% for the top (above C1) and bridge (at the middle of the C1-C2 bond) sites, respectively, but these last reductions are lower (11.3 and 13.8%, respectively) in the case of dihydrogen adsorption. In other words, after hydrogen exposition there is a weakening of the C-Pd interactions respect to the free-hydrogen Pd-doped graphene, which is more significant when atomic hydrogen is present. Although the binding energy of hydrogen on the Pd/graphene system is higher than the binding energy of Pd on graphene, there is still a C-Pd bonding after H2 adsorption (see Table 3), so the Pd decoration remains on the graphene surface. At the same time, the C-C bonds near the adsorption point are strengthened to a lesser extent (1.4 and 1.1% for the atomic and molecular hydrogen adsorption), compared with the free-hydrogen functionalized graphene. Nevertheless, due to the Pd decoration these C-C bonds are weaker than those in the bare graphene. The reason for these 4319



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Table 3. Overlap Population for Selected Bonds in Free PdH and PdH2, Bare Graphene, and Pd, PdH, and PdH2 GrapheneSupported Systems bondsa C-C system

site

C1-C3

C1-C4

H-Pd

C1-Pd

C2-Pd

H1-Pd

H-H

H2-Pd

H1-H2

PdH (free)

-

-

-

-

-

-

0.668

PdH2 (free)

-

-

-

-

-

-

0.155

graphene (bare) graphene/Pd

top

0.182

0.042

-

-

-

bridge

0.978

0.996

0.990

0.139

0.139

-

-

-

graphene/Pd/H

top

0.994

0.995

0.989

0.154

0.032

0.660

-

-

bridge

0.992

1.001

0.995

0.117

0.116

0.659

-

-

top

0.993

0.992

0.988

0.161

0.033

0.111

0.112

0.734

bridge

0.989

1.000

0.994

0.120

0.119

0.113

0.113

0.730

graphene/Pd/H2 a

C1-C2

C-Pd

1.012 0.985

1.012 0.985

1.008 0.979

-

-

0.155

0.673

See atom labeling in Figure 2.

Table 4. Electron Occupation for Selected Atomic Orbitals in Free PdH and PdH2, Bare Graphene, and Pd, PdH, and PdH2 Graphene-Supported Systems atomic orbitala C1 system

2s

2pz

2s

C3 2pz

2s

C4 2pz

2s

Pd 2pz

H1

H2 1s

5s

5pz

4dz2

1s

PdH (free)

-

-

-

-

-

-

-

-

-

0.505

0.081

1.078

1.337

PdH2 (free)

-

-

-

-

-

-

-

-

-

0.227

0.000

1.944

0.943

graphene (bare)

-

1.167

0.998

1.167

0.992

1.167

0.990

1.167

0.998

graphene/Pd

top bridge

1.179 1.179

0.901 0.914

1.173 1.179

0.951 0.910

1.173 1.171

0.949 0.977

1.173 1.171

0.960 0.985

0.248 0.250

0.000 0.000

1.934 1.946

graphene/Pd/H graphene/Pd/H2 a

site

C2

-

-

-

0.943

-

-

-

-

top

1.181

0.939

1.174

0.960

1.174

0.958

1.174

0.968

0.609

0.169

1.885

1.428

-

bridge

1.180

0.941

1.180

0.937

1.171

0.980

1.171

0.988

0.606

0.167

1.896

1.431

-

top

1.182

0.932

1.174

0.959

1.175

0.953

1.174

0.969

0.338

0.000

1.888

0.964

0.963

bridge

1.177

0.941

1.176

0.938

1.170

0.978

1.170

0.988

0.326

0.000

1.912

0.960

0.960


changes is the formation of new H-Pd bonds, particularly strong during the H adsorption process. The preceding bonding analysis supports the idea of a mechanism for hydrogen accumulation in decorated graphene sheets where the Pd particles work as a “bypass” for hydrogen transfer to carbon surface, dissociating the H2 molecule and supplying atomic H at the Pd-C interface. However, the suggested mechanism does not necessarily mean that the graphene layer is inactive. A previous theoretical study by Yang and Yang38 has shown adsorption of hydrogen atoms on graphite basal plane sites is exothermic and stable, while Roland et al.78 have proposed that the hydrogen spillover process can activate a previously low active support and induces subsequent hydrogen physisorption. Thus, the hydrogen uptake of TM-doped graphene could be interpreted in terms of atomic hydrogen spillover and adsorption, according with the experimental evidence involving others nanostructured carbon materials.21-24,29-33 3.3. Orbital Bonding Contributions. In this last section, we study the contribution to the adsorption process of the H, C, and Pd atomic orbitals, in order to get more insight into the bonding mechanism between atomic or molecular hydrogen and the

Pd-functionalizated graphene surface. The corresponding electron occupations for the atomic orbitals with an important participation in the adsorption are displayed in Table 4, whereas the overlap populations belonging to the main interactions between atomic orbitals are listed in Table 5. The results shown in Table 4 reveal that after Pd decoration the electron orbital occupation of the C 2pz orbital decreases significantly, about 8.4% for C1 and C2, which form the bridge site, and 9.4% for the adsorption onto C1. At the same time, C 2s orbital populates to a lesser extent, up to 1.1% for the mentioned C atoms. In the same way, Table 5 shows that the C 2pz-2pz interactions are weakened and contrarily the C 2s-2s ones strengthened after Pd adsorption. These changes are significantly smaller for the neighbor C atoms (see Tables 4 and 5), because their distances to the Pd atom are longer; the atomic orbital behaviors of C5 and C6 during the adsorption in the bridge site are similar to those of C3 and C4, respectively. The preceding results indicate that the C 2pz orbital is the major contributor for the C-Pd interaction, which is stronger when the decoration is located directly above the C atom. Previous theoretical79-81 studies also suggest that C pz orbitals hybridize to a great extent 4320



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Table 5. Overlap Population for Selected Atomic Orbital Interactions in Free PdH and PdH2, Bare Graphene, and Pd, PdH, and PdH2 Graphene-Supported Systems atomic orbital interactionsa C1-C2 system

site

C1-Pd

C2-Pd

H1-Pd

2s-2s 2pz-2pz 2s-2s 2pz-2pz 2pz-5s 2pz-5pz 2pz-4dz2 2pz-5s 2pz-5pz 2pz-4dz2 1s-5s 1s-5pz 1s-4dz2

PdH (free)

-

-

-

-

-

-

-

-

-

-

-

0.463

0.128

0.077

PdH2 (free)

-

-

-

-

-

-

-

-

-

-

-

0.132

0.000

0.000

graphene (bare) graphene/Pd

top

0.123

0.018

0.033

0.037

0.004

0.000

-

-

-

bridge

0.146

0.132

0.142

0.151

0.097

0.012

0.009

0.097

0.012

0.009

-

-

-

graphene/Pd/H

top

0.145

0.147

0.145

0.147

0.056

0.068

0.033

0.023

0.012

0.000

0.426

0.208

bridge

0.147

0.144

0.143

0.155

0.049

0.046

0.010

0.049

0.046

0.010

0.423

0.211

0.026

0.145

0.145

0.145

0.144

0.081

0.047

0.037

0.029

0.008

0.000

0.091

0.000

0.000

0.144

0.144

0.141

0.156

0.062

0.033

0.010

0.057

0.029

0.006

0.096

0.000

0.000

graphene/Pd/H2 top bridge a

C1-C3

0.136 0.144

0.175 0.138

0.136 0.144

0.176 0.138

0.026


with the d orbitals of the TM atoms. The bonding mechanism for graphene/Pd seems to be very similar to the Dewar, Chatt, and Duncanson model:46 the π orbital of carbon surface donates electrons into the Pd d states, while Pd back-donates charge density from the dz2 orbital to the antibonding orbital π*. Although it is well-known that the chemistry of most TM is dominated by the d orbitals, the bonding contributions of metal s and p orbitals are also important in many cases, in an analogous way to their role in organometallic compounds. In this way, the lobes of the C 2pz orbital (perpendicular to carbon surface) are well oriented to overlap with the Pd 5s, 5pz, and 4dz2 orbitals. Table 5 shows that the C(2pz)-Pd(4dz2) interaction is less than 6.5 and 18.1% of the overall C-Pd OP value for the bridge and top sites, respectively, with the main C-Pd overlap coming from the interaction between C 2pz and Pd 5s orbitals. When atomic hydrogen is present, it can be seen that the electron occupations of the Pd 5s, 5pz, and 4dz2 orbitals undergo a substantial variation for both adsorption sites, as a consequence of the formation of a strong H-Pd bond (see Table 4). Thus, this bond is essentially due to overlaps between the H 1s and the Pd 5s, 5pz, and 4dz2 orbitals, as in the isolated linear PdH structure. However, Table 5 shows that the OP values for the H(1s)Pd(5s) and H(1s)-Pd(4dz2) interactions decrease during the adsorption of the PdH system, because Pd 5s and 4dz2 orbitals also participate in the C-Pd bonding. Similarly, the C(2pz)Pd(5s) interaction is weaker than in the free-H decorated graphene, and the binding between C 2pz orbitals is stronger. The behavior of the Pd 5pz orbital is different. In effect, we can see from Table 5 that in the graphene/Pd/H system the H(1s)Pd(5pz) interaction constitutes about 31.5% of the H-Pd overlap, while the value is almost 19.2% for the free PdH. The behavior of the C(2pz)-Pd(5pz) contribution to the overall CPd OP is similar and could be attributed to the significant enhancement in the electron occupation of the metal 5pz orbital after H adsorption. Finally, the results in Table 4 and 5 show that only Pd 5s orbital experiences a considerable increase in the electron orbital occupation during H2 adsorption and the main contribution to the H-Pd bonds is due to the H(1s)-Pd(5s) overlap. This orbital interaction is until 45% larger in the unsupported PdH2 complex, where the metal 5s orbital is not involved in the surface bonding. On the other hand, the H(1s)-Pd(5s) and C(2pz)-

Figure 4. Total DOS curves of free PdH (a), graphene-supported PdH (b), free PdH2 (c), and graphene-supported PdH2 (d). The shaded areas correspond to the contributions of the Pd and H atoms.

Pd(5s) OP values are smaller and larger, respectively, than those found during the H adsorption process. We could conclude thus that dihydrogen interacts to a lesser extent with the Pd decoration; in such a way the metal is bonded stronger with the graphene sheet. We also performed DOS curves in order to confirm the previous findings about the orbital contributions to the Pd-H bonding. In Figure 4, the DOS plots for free and graphenesupported PdH and PdH2 systems are shown. The shaded areas correspond to the contributions of the Pd and H atoms, magnified by a factor of 2 in the adsorbed state (Figure 4b,d). We selected the adsorption geometries belonging to the bridge site, but similar curves were obtained during the adsorption on the top site. The isolated PdH DOS (see Figure 4a) presents a very sharp band at -14.2 eV, which mainly comes from H 1s orbital. The orbital composition analysis of this state indicates the contributions are 65.5% H 1s, 20% Pd 5s, 3.5% Pd 5pz, and 11% Pd 4dz2, approximately, verifying that the principal Pd-H bonding is due to the H(1s)-Pd(5s) interaction. States at 7.2 and 12.1 eV (this last not shown) also belong to H 1s (5 and 26.5%, respectively), Pd 5s (43.5 and 26.5%), Pd 5pz (50.5 and 44.5%), and Pd 4dz2 (1 and 2.5%) orbitals. The intense band at about -11.4 eV corresponds mainly to Pd 5s þ 4d orbitals, while at -5.6 eV there are only Pd 5px þ 5py contributions. Figure 4b shows the DOS of PdH adsorbed on the graphene surface in the bridge site. The band at lower energies is again due mainly to H 1s (61.5%) and Pd 5s (24%) orbitals and almost does not suffer modifications, but the others states undergo some hybridization after adsorption on the carbon surface. 4321


The Journal of Physical Chemistry C The DOS plot for free PdH2 Kubas complex is shown in Figure 4c. The orbital contributions of the peak at -16.6 eV are 45% each H 1s, 7% Pd 5s, 1% Pd 5pz, and 2% Pd 4dz2, and so the H(1s)-Pd(5s) interaction is again mainly responsible for the Pd-H bond, though in this case it is less stronger than that of the isolated PdH system. The stabilization of this state at lower energies is due to the repulsive interaction of both H 1s orbitals. The H2 σ* antibonding orbital is located at much higher energies and is not shown here. The DOS also exhibits an intense broadening around -11.5 eV, which comes from the Pd 5s þ 4d orbitals. The empty state at -7.5 eV corresponds to H 1s and Pd 5s, 5pz and 4dz2 orbitals, like another one at 4.8 eV (not shown), while the two peaks between -5.5 and -6.5 eV are mainly originated in Pd 5px þ 5py orbitals. Looking at the Pd and H projections in Figure 4, c and d, it can be seen that the empty states are hybridized and pushed to higher energies after their interaction with the bridge site of the graphene surface. The participation of the Pd 5pz orbital in the peak at -16.6 eV is much lower in the adsorption case. Notice that all these observations are in complete correlation with the previous orbital-orbital OP discussion. The bonding between the graphene-supported Pd atom and dihydrogen molecule can be understood by means of the Kubas coordination,47 where two cooperatively coupled intermolecular donor-acceptor delocalizations lead to overall charge reorganization: the σ donation, from H2 σ orbital to the Pd 5s orbital, and the π back-donation, from the filled Pd 4dyz orbital to the H2 σ* orbital. Our calculations show that occupancy of the H2 σ bonding orbital decreased from 2e to 1.849e on going from free H2 molecule to the unsupported PdH2 system, but during adsorption on Pd-decorated graphene this molecular orbital is slightly populated (to 1.894e and 1.898e for the bridge and top sites, respectively), as a result of the C-Pd bonding. In the same way, the σ* antibonding orbital of H2 molecule reduced its electron population when the PdH2 complex is adsorbed on the graphene layer (0.038e versus about 0.027e). Thus, both the σ donation and π back-donation are decreased in the graphenesupported PdH2 system, which is in line with shortening and strengthening of H-H bond observed during our geometrical and OP analysis. Additionally, it should be noted that the reported Pd binding energy to graphene (∼1.0 eV) is much lower than the cohesive energy of Pd (∼3.8 eV), and so the possibility of clustering will be a serious issue. Our preliminary calculations in the framework of the present method suggest that the adsorption of two individual Pd atoms on graphene is stronger when they occupy near adsorption sites, originating a parallel Pd2 dimer with respect to the carbon surface. In this way, individual Pd atoms tend to aggregate on a graphene surface and they may not be further accessible for the adsorption of hydrogen molecules. We are currently investigating the Pd clustering effects on the hydrogen-bonding mechanism and the results will be published elsewhere.

4. CONCLUSIONS In the present study, we have analyzed the adsorption of dihydrogen on a Pd-functionalized graphene surface by means of GGA-DFT calculations, considering different molecular and dissociative PdH2 coordination structures. The most stable adsorption geometry corresponds to a side-on approach mode, similar to that present in the isolated so-called Kubas complex, with a relaxed H-H bond and an H-Pd-H angle of

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28 approximately. Comparable binding energies were obtained for the adsorption on the bridge (at the center of a C-C bond) and top (above a C atom) sites. Using OP and COOP analysis, we show that during the adsorption process C-Pd bonds are formed, with an important role of the C 2pz and Pd 5s, 5pz, and 4dz2 orbitals, while C-C bonds near the adsorption sites are weakened. We also verified that the bonding between the carbonsupported Pd particles and dihydrogen molecule are based on the Kubas coordination model and leads to a strengthened H-H bond with respect to the free PdH2 complex. However, the H-Pd interactions developed during the molecular adsorption are clearly weaker than that originated in the atomic hydrogen bonding. These findings are in concordance with previous theoretical and experimental studies and support the proposal that hydrogen uptake of Pd-doped graphene-based materials can be related with two possible nonexclusive mechanisms: dihydrogen adsorption to form Kubas PdH2 complexes, and/or dissociative chemisorption on the Pd decoration and subsequent H spillover on the carbon sheet. However, since that Kubas complexes are formed by individual Pd atoms, further work is clearly needed to understand the Pd clustering action on the bonding mechanism for hydrogen storage.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (þ54) 291 4595142. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank M. I. Rojas and E. P. M. Leiva for their advice during the SIESTA program implementation. Our work was supported by SGCyT-UNS and CONICET PIP 2009, PICT 1186, 560, and R656. I.L.-C. and E.G. are fellows of CONICET. A.J., M.A.V., and G.P.B. are members of that Institution. We also acknowledge useful reviewers’ comments. ’ REFERENCES (1) Iijima, S. Nature 1991, 354, 56–58. (2) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes. Synthesis, Structure, Properties, and Applications; Springer: Berlin, 2001. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (4) Ferre-Vilaplana, A. J. Chem. Phys. 2005, 122, 104709. (5) Lin, Y. M.; Jenkins, K. A.; Valdes-Garcia, A.; Small, J. P.; Farmer, D. B.; Avouris, P. Nano Lett. 2009, 9, 422–426. (6) Zanella, I.; Fagan, S. B.; Mota, R.; Fazzio, A. J. Phys. Chem. C 2008, 112, 9163–9167. (7) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377–379. (8) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 2307–2309. (9) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127–1129. (10) Pradhan, B. K.; Harutyunyan, A. R.; Stojkovic, D.; Grossman, J. C.; Zhang, P.; Cole, M. W.; Crespi, V.; Goto, H.; Fujiwara, J.; Eklund, P. C. J. Mater. Res. 2002, 17, 2209–2216. (11) Ma, Y. C.; Xia, Y. Y.; Zhao, M. W.; Ying, M. J. Phys. Rev. B 2002, 65, 155430. (12) Shiraishi, M.; Takenobu, T.; Yamada, A.; Ata, M.; Kataura, H. Chem. Phys. Lett. 2002, 358, 213–218. 4322


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DFT Study of Hydrogen Adsorption on Palladium Decorated Graphene

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